Field of the Invention
The invention relates to a method and a base station system for voice transmission via a radio interface in a digital radio communication system, in particular in a digital mobile radio system or in a wireless subscriber access system or access network system.
A base station system is a part of a digital radio communication system which may correspond, for example, to the GSM mobile radio network (Global System for Mobile Communications), as is known, inter alia, from the publication xe2x80x9cMobilfunk und intelligente Netzexe2x80x9d [Mobile radio and intelligent networks], by J. Biala, Vieweg Publishers, 1995, in particular pages 57 to 92. In particular, the radio communication system may alternatively correspond to a third generation mobile radio system (UMTSxe2x80x94Universal Mobile Telecommunications System) which in general will have the same system configuration as the GSM mobile radio network, or an access network system as is known from the article xe2x80x9cDrahtlos zum Freizeichenxe2x80x9d [Wireless to the call connected signal], by M. Reixcex2, telcom report 18, 1995, pages 34 to 37.
Such radio communication systems allow communication links to be set up to transmit information, in particular voice information, via a radio interface between base stations and subscriber radio stations. The radio stations may in this case be configured, for example, as mobile stations in a mobile radio system, or as wireless network access units in an access network system.
Various methods are used for subscriber separation. These methods are generally based on a frequency-division multiplexing method FDMA (Frequency Division Multiple Access) with a given frequency bandwidth being split up into a plurality of frequency channels for use. If a plurality of subscribers on a common carrier frequency in the radio interface are separated by different timeslots, then a time-division multiplexing method TDMA (Time Division Multiple Access) is also involved, as is also used, for example, in the GSM mobile radio system. If the subscribers on the same carrier frequency are separated by different codes, then a code-division multiplex method (CDMAxe2x80x94Code Division Multiple Access) is involved, as is known, inter alia, from the article xe2x80x9cEffizienter Teilnehmerzugriff fxc3xcr 3. Generation der Mobilkommunikationxe2x80x9d [Efficient subscriber access for 3rd generation mobile communication], by T. Ketseglou, T. Zimmermann, telcom report 16, 1993, pages 38 to 41. Such a CDMA method is intended for both, second and third generation mobile radio systems and for access network systems. Furthermore, Published Non-Prosecuted German Patent Application DE 195 49 148 discloses a hybrid of these two methods for subscriber separation, which has time-division multiplex subscriber separation in addition to CDMA subscriber separation.
A radio communication system includes at least one base station system which contains, for example, a base station controller which is connected to a plurality of base stations. The base stations each supply a radio area, also called a radio cell, with radio resources. In this case, each base station may have only a limited supply of radio resources, in order to avoid interference. The radio areas of adjacent base stations overlap at the boundary regions of the radio cells or as a result of the radio cells being formed hierarchically, as is planned for second and third generation mobile radio systems. In mobile radio systems, the connection of a plurality of base stations to one base station controller allows a handover procedure between two base stations in order to make it possible for a mobile subscriber to have unrestricted freedom of movement with his mobile station.
The base station controller in this case carries out the function of switching and management of the radio channels in the base station, and administration and implementation of handover procedures.
In digital radio communication systems, digital voice codecs are used for coding voice information. In a GSM mobile radio system, these voice codecs include, for example, a voice coder and a downstream channel coder. In the voice coder, the 64 kbit/s data rate of PCM30 channels is reduced, for example, to a data rate of 13 kbit/s, which is called the net bit rate since it contains only the pure coded voice information. Additional redundancy is then added to the voice signal in the channel coder through the use of an error correction method, so that the bit rate is increased, for example, to 22.8 kbit/s, the gross bit rate. This example relates to a full-rate codec. As a further development, half-rate codecs have also been introduced in the GSM mobile radio system, which use only half the data rate for voice transmission. Such a compression of the voice signals is necessary since the available radio resources are limited and the aim is to supply as many subscribers as possible using the mobile radio system.
The proportion of error protection in the gross data rate (=net data rate of the wanted data+error protection) with the voice codecs in use is in general high, and is excessive if the transmission channel conditions are good. For this reason, the use of adaptive multirate voice codecs (AMRxe2x80x94Adaptive Multi Rate) is proposed, where the proportion of error protection is varied as a function of the transmission conditions on the radio interface. By reducing the error protection, it is possible to increase the bit rate after voice coding, and thus to improve the voice quality, or to reduce the gross data rate and thus create capacities for further subscribers.
The bit error rate determined by the receiving radio station may be used, for example, as a parameter for determining the error protection. However, this parameter has the disadvantage that the bit error rate can change very quickly as a result of short-term disturbances, such as shadowing by obstructions and, in particular, resulting from the movement of subscribers of mobile radio systems, so that the voice codec is unable to follow such a fast change. Furthermore, it is hardly possible to estimate the future transmission conditions from values measured in the past.
It is accordingly an object of the invention to provide a method and a base station system which overcome the above-mentioned disadvantages of the heretofore-known methods and systems of this general type and which allow improved voice and channel coding, irrespective of fluctuations, in particular short term variations, in the transmission channel characteristics.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for voice transmission via a radio interface in a digital radio communication system including a base station connected to a base station controller, and a radio station located in a radio coverage area of the base station. The method includes the steps of:
transmitting a signal via a radio interface between a radio station and a base station;
determining, from the signal, a characteristic value relating to transmission conditions of the radio interface;
storing the characteristic value in a storage device for providing a stored characteristic value;
performing, with a voice and channel codec, a voice and channel coding at a transmitter side for a voice transmission; and
controlling the voice and channel coding at the transmitter side by taking into account a currently determined characteristic value and the previously determined stored characteristic value.
In other words, in the method according to the invention for voice transmission via a radio interface in a digital radio communication system which has at least one base station which is connected to a base station controller, as well as at least one first radio station which is located in the radio coverage area or radio supply area of the base station, at least one signal is transmitted via the radio interface between the first radio station and the base station. At least one characteristic value relating to the transmission response of the radio interface is determined from this signal. The characteristic value is stored in at least one storage device and is taken into account at a later point in time in addition to a currently determined characteristic value for controlling transmitter-side voice and channel coding through the use of at least one voice and channel codec.
This characteristic value may be related, in a first embodiment of the invention, to a reception level, a bit error rate and/or a value which is proportional to the signal propagation time between the first radio station and the base station, and/or a signal-to-noise ratio. Characteristic values, which can be obtained particularly easily from radio communication systems, are the reception level and the bit error rate (which are indicated as scaled values RXLEV, RXQUAL) since, as a rule, these are already available in current implementations.
In two alternative embodiments of the invention, the voice and channel codec is driven in such a way that, in the first embodiment, the bit rate of the error protection is varied in the channel codec, and the thus the gross bit rate at the output of the channel codec as well, with the net bit rate at the output of the voice codec being kept constant, and in such a manner that, in the second embodiment, the net bit rate at the output of the voice codec and the bit rate of the error protection in the channel codec are varied, with the gross bit rate at the output of the channel codec being kept constant.
The first of these two embodiments has the advantage that by reducing the error protection, the gross bit rate is reduced, and additional capacity is thus created for further voice transmissions on the radio interface. On the other hand, the second embodiment has the advantage that the voice coding can be configured to be more generous when there is less error protection, thus improving the voice quality.
According to a further embodiment, a statistical mean value or a difference is determined from the stored and the currently determined characteristic value, and this is taken into account in each case for controlling the voice and channel codec.
The radio coverage area or radio supply area of the base station is also advantageously split into geographical subareas, and the location or position of the radio station is determined and assigned to a geographical subarea. The location or position may in this case be determined through the use of a global localization system, such as the GPS (Global Positioning System) by directional antennas, range measurements and/or during the network planning of the radio communication system.
In a further embodiment of the invention, this assignment of radio stations to geographical subareas allows the determined characteristic value to be weighted by a weighting coefficient. This weighting coefficient is defined for the individual geographical subareas during network planning, and may vary as a function of geographical characteristics and/or time. Time variation may be worthwhile and advantageous, for example, due to the traffic level being increased as a function of the time of day, that is to say increased transmission disturbances, or as a result of a higher subscriber density. The weighting coefficient thus has the advantageous effect that, for example, more error protection is used in certain subareas and/or at certain times, in order to ensure a uniformly high transmission quality.
A combination of a plurality of geographical subareas which have the same or similar geographical characteristics to form a subarea and the definition of a common weighting coefficient is advantageous, for example, in rural regions, thus allowing the determination and storage of the characteristic values, and the control process, to be simplified.
In accordance with another mode of the invention, the characteristic value is determined periodically at given time intervals or when the radio station moves from one of the geographical subareas to another one of the geographical subareas.
In accordance with yet another mode of the invention, further characteristic values relating to the transmission conditions between the base station and further radio stations, which are located in the radio coverage area of the base station or the same geographical subarea as the radio station, are determined and stored in the storage device. The previously determined stored further characteristic values and currently determined characteristic values are used for controlling the voice and channel coding at the transmitter side for the voice transmission between the base station and the radio station.
With the objects of the invention in view there is also provided, a base station system in a digital radio communication system having a radio station. The base station system includes:
a base station covering a radio coverage area and having a transmitting/receiving device for transmitting and receiving voice information via a radio interface between the base station and the radio station located in the radio coverage area of the base station;
a base station controller connected to the base station;
a signal evaluation device for determining a characteristic value from a signal transmitted via the radio interface, the characteristic value relating to transmission conditions of the radio interface;
a storage device storing the characteristic value for providing a stored characteristic value;
a voice and channel codec associated with the base station controller, the voice and channel codec performing a transmitter-side voice and channel decoding for a voice transmission via the radio interface; and
a control device operatively connected to the voice and channel codec for controlling the transmitter-side voice and channel decoding as a function of the stored characteristic value having been determined previously and the characteristic value having been determined currently.
In a further advantageous embodiment, a three-dimensional memory matrix is provided in the storage device, in which the characteristic value is entered on the basis of the geographical subarea in which the first radio station is located, and on the basis of time. Together with the weighting coefficients which are in each case stored for the geographical subarea, thus all the data required for an optimum control of the voice and channel codec is available.
The characteristic value can be determined and stored at predetermined time intervals, periodically, controlled by a timer. In a situation where the first radio station is in the form of a mobile radio station, the characteristic value may also be stored on changing to a different geographical subarea of the radio coverage area of the base station. The control of the voice and channel coding may likewise be initiated on the basis of the same criteria.
These embodiments advantageously make it possible to take account of empirical values, that is to say periodically stored characteristic values, which for instance depend on a position and a time and which are, if expedient, in each case weighted by weighting coefficients, for controlling the voice and channel codec. This advantageously smoothes out short-term disturbances in the transmission quality, and the voice coding can be carried out, for example, with an optimally matched error protection.
In accordance with another feature of the invention, the control device weights the characteristic value by a weighting coefficient, which is defined for each of the geographical subareas and varies as a function of at least one of geographical characteristics and time.
In two further embodiments of the invention, the control of the transmitter-side voice and channel coding takes account not only the stored and currently determined characteristic values for the voice transmission between the base station and the first radio station, but also stored and currently determined characteristic values relating to the transmission response between the base station and further radio stations. These further radio stations are in this case likewise located in the radio coverage area of the base station, or in the same geographical subarea as the first radio station.
This advantageously provides a database in which characteristic values are stored of all the radio stations located in the radio coverage area of the base station, on the basis of position and time. In this case, for a mobile radio station which, for example, is entering a new geographical subregion, it is possible to access the characteristic values of other radio stations which are located or have been located in that subregion, for controlling the voice and channel codec, in order in this way to provide voice and channel coding that is as optimal as possible. As a result of these measures, the control of the voice and channel coding becomes highly insensitive to short-term disturbances in the transmission quality. Disturbances of longer duration, on the other hand, are taken into account by regularly determining the characteristic values and, for example, their statistical mean value.
The following description of a plurality of exemplary embodiments of the method according to the invention and of the base station system according to the invention is only of an exemplary nature. The described features are not necessarily required in the described form in order to achieve the desired success.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and base station system for voice transmission via a radio interface in a digital radio communication system, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.